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Long-duration insurance contracts - Summary of Other Information Relating to Long-Duration Insurance Liabilities (Detail) - USD ($) $ in Millions | Sep. 30, 2025 | Sep. 30, 2024
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Periodic Payment Annuity [Member] |  |
Liability for Future Policy Benefit, Activity [Line Items] |  |
Undiscounted expected future benefits | $ 30,522 | $ 30,915
Weighted average discount rate | 5.70% | 5.20%
Weighted average accretion rate | 4.80% | 4.80%
Weighted average duration (in years) | 16 years | 17 years
Life and Health [Member] |  |
Liability for Future Policy Benefit, Activity [Line Items] |  |
Undiscounted expected future gross premiums | $ 103,457 | $ 101,134
Discounted expected future gross premiums | 61,069 | 58,530
Undiscounted expected future benefits | $ 94,059 | $ 91,751
Weighted average discount rate | 5.00% | 4.70%
Weighted average accretion rate | 2.60% | 2.60%
Weighted average duration (in years) | 13 years | 13 years

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